1. Field of the Invention
The present invention relates to a Doppler-effect speed detecting apparatus adapted to detect or determine the ground speed of a motor vehicle, utilizing the Doppler effect of waves.
2. Discussion of the Related Art
In a motor vehicle, the detection or determination of the running speed of the vehicle with high precision is required for achieving adequate control of the vehicle motions in relation to the vehicle running speed, for example, for effecting anti-lock regulation of braking forces of the wheels or traction control of the drive wheels. To this end, it is a conventional practice to estimate the vehicle running speed on the basis of the rotating speeds of the wheels which are detected by respective speed sensors provided for the individual wheels. An anti-lock or traction control of a braking system to regulate the brake forces or drive forces applied to the wheels is implemented in special running conditions of the vehicle in which the slip ratio of the wheels is considerably higher than in an ordinary running condition. In such special running conditions, therefore, the accuracy of estimation of the actual running speed of the vehicle based on the speeds of the wheels is limited.
In light of the drawback encountered in detecting the vehicle running speed using the wheel speed sensors, it has been proposed to provide a motor vehicle with a Doppler-effect vehicle ground-speed detecting apparatus, which is capable of detecting the vehicle running speed with improved accuracy irrespective of an increase in the slip ratios of the wheels. Generally, such a Doppler-effect ground-speed detecting apparatus has (a) a transmitter for transmitting a wave toward the ground or road surface, (b) a receiver for receiving a portion of the transmitted wave which is reflected by the ground surface, and (c) a signal processor or ground-speed determining means for determining the ground speed of the vehicle, according to the Doppler effect or shift, on the basis of the frequencies of the waves as transmitted from the transmitter and as received by the receiver (hereinafter referred to as "transmitted and received waves").
However, the Doppler-effect ground-speed detecting technique suffers from inherent drawbacks not encountered in the prior art which utilizes the wheel speed sensors to estimate the ground speed of the vehicle. That is, since a Doppler-effect ground-speed detecting apparatus utilizes a wave that is irregularly reflected by the road surface, the detecting accuracy of this apparatus is affected by the road surface condition, more precisely, the wave reflecting condition of the road surface. Described in detail, the component of the transmitted wave to be irregularly reflected by the road surface is reduced with a result of lowering the output level of the receiver, when the road surface on which the vehicle is running has a relatively high degree of smoothness, for instance, when the road surface is covered with water or ice. The lowering of the output level of the receiver leads to undesirable reduction in the accuracy of detection of the ground speed of the vehicle.
To assure high accuracy of detection of the vehicle ground speed even when the output level of the receiver is comparatively low, one of the assignees of the present application proposed a Doppler-effect ground-speed detecting apparatus as disclosed in JP-A-4-212086. This apparatus includes (a) vehicle speed estimating means for obtaining an estimated vehicle speed on the basis of the speeds of the vehicle wheels, (b) receiver-output detecting means for detecting the output level of the receiver of the Doppler-effect speed sensor, and (c) ground-speed determining means for determining the vehicle ground speed on the basis of the frequencies of the transmitted and received waves when the output level of the receiver detected by the receiver-output detecting means is higher than a predetermined threshold value, and determining as the vehicle ground speed the estimated vehicle speed obtained by the vehicle speed estimating means when the detected output level of the receiver is not higher than the threshold value.
The inventors of the present invention have continued efforts to pursue techniques for detecting the ground speed of a motor vehicle with sufficiently high accuracy even when the output level of the receiver is comparatively low. As a result, the present inventors have recognized a fact that there exists a given correlation between the actual vehicle ground speed, and the original vehicle ground speed as determined by the ground-speed determining means on the basis of the frequencies of the transmitted and received waves (hereinafter referred to as "originally determined vehicle ground speed"), irrespective of the output level of the receiver as compared with a predetermined threshold. Described more specifically, the originally determined vehicle ground speed is lower than the actual vehicle ground speed when the output level of the receiver is relatively low, in the case where the transmitter and receiver are attached to the vehicle body such that the boresights or optical axes of the transmitter and receiver are inclined in the backward or reverse running direction of the vehicle. The amount of decrease of the originally determined vehicle ground speed with respect to the actual vehicle ground speed increases with an increase in the actual ground speed. In the case where the boresights of the transmitter and receiver are inclined in the forward running direction of the vehicle, the originally determined vehicle ground speed is higher than the actual vehicle ground speed when the output level is relatively low. The amount of increase of the originally determined vehicle ground speed with respect to the actual vehicle ground speed increases with an increase in the actual ground speed. In view of this fact, it was found that the originally determined vehicle ground speed could be effectively utilized, rather than ignored in the prior art, to determine the actual vehicle ground speed with high accuracy even when the output level of the receiver is relatively low due to insufficient irregular reflection of the transmitted wave from the road surface under certain conditions of the road surface.